Maintain CSRF authenticity token - Fixes #171

This is a more focused solution that what was proposed #165, internally keeping the CSRF token current (if it exists) instead of requiring the developer to add a data attribute.  

My reasoning behind wanting to implement this is the fact that the **jquery_ujs** driver explicitly reads from the csrf-token meta tag and uses it when setting the `X-CSRF-Token` request header and also in a hidden field of the form that's created and submitted when a link with `data-method` defined is clicked.  This is the underlying cause of the use case described in #171, and keeping the meta tag up-to-date would resolve it.  

Looking for some feedback on this.

/cc @satb @macournoyer @dhh